Title:A Hyper Suprime-Cam View of the Interacting Galaxies of the M81 Group

Abstract: We present the first results of a wide-field mapping survey of the M81 group
conducted with Hyper Suprime-Cam on the Subaru Telescope. Our deep photometry
reaches $\sim2$ magnitudes below the tip of the red giant branch (RGB) and
reveals the spatial distribution of both old and young stars over an area of
$\sim 100\times115$ kpc at the distance of M81. The young stars ($\sim30-160$
Myr old) closely follow the neutral hydrogen distribution and can be found in a
stellar stream between M81 and NGC\,3077 and in numerous outlying stellar
associations, including the known concentrations of Arp's Loop, Holmberg\,IX,
an arc in the halo of M82, BK3N, and the Garland. Many of these groupings do
not have counterparts in the RGB maps, suggesting they may be genuinely young
systems. Our survey also reveals for the first time the very extended ($\geq
2\times \rm{R_{25}}$) halos of RGB stars around M81, M82 and NGC\,3077, as well
as faint tidal streams that link these systems. The halos of M82 and NGC\,3077
exhibit highly disturbed morphologies, presumably a consequence of the recent
gravitational encounter and their ongoing disruption. While the halos of M81,
NGC\,3077 and the inner halo of M82 have the similar $(g-i)_{0}$ colors, the
outer halo of M82 is significantly bluer indicating it is more metal-poor.
Remarkably, our deep panoramic view of the M81 group demonstrates that the
complexity long-known to be present in HI is equally matched in the low surface
brightness stellar component.